Surgical heart valve replacement may involve implantation of one of three distinct prosthesis types; mechanical (synthetic), bioprosthetic (chemically-fixed porcine valve or bovine pericardium), or human allograft. These prostheses provide effective hemodynamic improvement for replacement of native aortic valves that are either congenitally malformed or have been damaged by degenerative changes or disease resulting in either aortic insufficiency or aortic stenosis. Of the approximately 55,000 aortic valve implants annually in the U.S., 75% are mechanical valves. The remainder of the replacements are of transplanted tissues. Of these, over 80% are porcine bioprostheses; the relatively small number of allografts (2,500 per year) is primarily due to their limited availability.
The criteria for an ideal prosthesis would include natural hemodynamics, long-term durability, low incidence of thromboembolic complications, freedom from calcification, proven lack of immunogenicity and no inappropriate hyperplastic responses following implantation. Even in autologous transplant situations, the surgical handling of the tissue, such as vein grafts, may itself be a stimulus for tissue hyperplasia and subsequent failure of the graft.
In accordance with this invention, heart valves, pulmonary and aortic, may be prepared having advantageous properties with respect to wear, tendency to calcify, stimulation of immune responses, and reduced difficulty in acquisition. It is also applicable to other forms of tissues particularly those composed of structural interstitial collagens.
Various synthetic grafts and mechanical organs have been developed and are currently in use. However, these synthetic replacements are known to be subject to embolic complications or decreases in material strength over long periods of implantation. Although structural modifications in mechanical prostheses such as heart valves have been improved with respect to their wear characteristics, they remain liable to valve malfunction, which may occur suddenly and without warning, resulting in emergency situations requiring surgical intervention and replacement of the artificial prosthetic device. Because of the surface properties of synthetic/mechanical prostheses used in the vasculature, platelet adhesion increases the likelihood of thrombus formation and anticoagulant therapy must be provided for the life of the implant and makes such implants undesirable for certain groups of potential recipients (for example, women of child-bearing years).
One alternative, bioprosthetic heart valves, are prepared from valve tissues of porcine or bovine origin. Because these are species discordant immunologically from man, they are rapidly rejected by the implant recipient despite the use of immunosuppression drug therapy that would otherwise maintain an allograft. Significantly, these tissues are liable to hyperacute rejection by the recipient because of the presence in the recipient of preformed natural antibodies which recognize antigens on the surface of foreign cells, particularly those of the endothelial lining of heart valves and blood vessels. While bovine or porcine valve tissues are structurally and biomechanically appropriate for use in humans, the potential of such foreign tissue to stimulate immune rejection in the recipient has in the past dictated treatments with chemical cross-linking agents such as glutaraldehyde. Such treatment of the tissue reduces the stimulation of an immunological response by the recipient to the foreign tissue, and also stabilizes the collagen protein of the resulting non-viable valve tissue making it more resistent to degradation by proteolytic enzymes. However, because these tissue grafts are non-viable, there is no biosynthetic mechanism to repair structural proteins broken down during the operation of the tissue in the recipient. Such tissue grafts tend to calcify with time, increasing the risk of structural damage and consequential failure. While occurring with less frequency relative to mechanical grafts, thromboembolism is also a patient management issue for recipients of these grafts.
Similarly, organs such as kidneys have been transplanted allogeneically from one sibling to another in an effort to minimize immunologically mediated reactions in the transplant recipient, which would result in organ rejection. These patients, as well as patients receiving transplant organs from donors other than a sibling, are frequently administered drugs to suppress their immune system. While the immunological response to transplant tissue or organs may be suppressed through the use of immunosuppressant drugs to minimize rejection, immunosuppressant therapy is general in nature. Hence, immunosuppressant drugs also tend to suppress the immune response generally, which reduces the transplant recipient's ability to combat infection.
More recently processes have been described for generating improved bioprosthesis for human or mammalian implant, by treatment of non-human tissue. See, Orton, E. Christopher, U.S. Pat. No. 5,192,312. Orton discloses generation of implant tissue by removing native cells from tissue of, for example, porcine origin; and then, repopulating the tissue with new cells in the presence of growth factor. The repopulating cells are immunologically compatible with the intended implant recipient. The biografts produced by this process are free from many of the disadvantages of other prior art bioprosthesis.